Zoom lens

ABSTRACT

The present invention provides a small, thin, light and low cost zoom lens suitable for cellular telephones, portable information terminals, etc. In particular, it can provide a small and thin, high performance zoom lens having a zoom ratio of approximately 2, a depth of less than 9 mm during shooting and in storage, a total lens length of less than 30 mm, an angle of view of approximately 61°, and a F number of approximately 2.8 providing a sufficient light, with various aberrations all suitably corrected.

FIELD OF THE INVENTION

[0001] The present invention relates to a zoom lens suitable for smalldigital still cameras, video cameras and the like equipped with imagepickup devices such as CCD and the like, in particular, a zoom lenssuitable for small digital still cameras, video cameras and the likebuilt into cellular telephones, portable information terminals (PDA),etc.

BACKGROUND ART

[0002] In recent years, due to remarkable technical advancements insolid state image pickup devices for uses in digital still cameras,video cameras and the like, small charge-coupled devices (“CCD”) andsimilar devices are developed and, with it, a demand of smaller andlighter optical systems are in great demand.

[0003] In particular, there is a need for smaller and thinner opticalsystems to be used on cellular telephones and portable informationterminals as they become smaller and thinner. The optical systems usedon the cellular telephones and portable information terminals of theprior art have been relatively small and suitable for demands forsmaller and thinner units because they were fixed focal point lenssystems.

[0004] In order to have a zoom lens that provides variable magnifyingpower on a cellular telephone and a portable information terminal wherea smaller and thinner unit is mandatory, it is necessary to have aplurality of lens barrels that are arranged to be able to slide in andout and cause them to collapse into the body when it is not in use inorder to make the system thinner. The embodiment of the lens barrels,including the collapsible mount mechanism, becomes more complex as thenumber of components increases.

[0005] In order to improve the above situation, the present inventionintends to provide a small, thin, and light zoom lens having a highquality optical capability suitable for being used on cellulartelephones and portable information terminals, more specifically, a zoomlens having a zoom ratio of about 2, a depth direction stroke in theincidence direction of the object light between the in-use and thenot-in-use (stored) conditions of less than 9 mm, and the longestdimension when it is in-use of less than 30 mm.

[0006] The zoom lens of the present invention comprises: a first lensgroup having a negative refractive power as a whole, a second lens grouphaving a negative refractive power as a whole, and a third lens grouphaving a positive refractive power as a whole, arranged in said orderfrom object side to image plane side, for zooming from a wide-angle endto a telephoto end by means of moving said third lens group from imageplane side to objection side as well as correcting image plane changesrequired, in accordance with said zooming by means of moving said secondlens group; wherein said first lens group consists of a lens having anegative refractive power and a prism for changing a light path arrangedin said order from the object side.

[0007] Since the depth dimension of the zoom lens according to saidembodiment is the depth, dimension in the direction the object lightenters into the first lens group (a lens and a prism), it is possible toobtain a thin and small zoom lens wherein the depth dimension and thedimension between the first lens group to the image plane remainconstant regardless of whether it is used or not for shooting.

[0008] In the above embodiment, it is possible to adopt such anembodiment wherein the second lens group consists of a lens with anegative refractive power and an aperture stop exists between the secondlens group and the third lens group. In this embodiment, the totallength in the optical axis direction becomes shorter and the lens groupson both sides (located on the upstream side and the downstream side) ofthe aperture stop can be formed in such a way as to have approximatelyidentical external dimensions, so that the zoom lens can be made morecompact efficiently.

[0009] In the above embodiment, it is possible to adopt such anembodiment wherein the lens of the first lens group has an asphericalsurface, the aspherical surface is formed on the surface with a smallercurvature radius, and the negative aspherical surface has a negativerefractive power weakening toward its periphery. According to theseembodiments, a better optical characteristic can be achieved as variousaberrations can be easily corrected by having an aspherical surface, anddistortion can be more easily corrected by having the aspherical surfaceon the surface with a smaller curvature radius and forming it in such away as to make the refractive power weaken toward the periphery.

[0010] In the above embodiment, said third lens group can be constitutedto have at least one lens with a positive refractive power and at leastone lens with a negative refractive power. According to said embodiment,various aberrations can be corrected with a better balance.

[0011] In the above embodiment, said third lens group can be constitutedto have a lens at a position closest to the object having a positiverefractive power and an aspheric surface at least on one side. Accordingto said embodiment, spherical aberration can be corrected most suitably.

[0012] In the above embodiment, the prism of said first lens group canbe formed to have an entrance surface and an exit surface both oblong ina direction perpendicular to a plane that includes an entrance axis andan exit axis. According to this embodiment, the zoom lens can be madethinner in the direction the object light enters (the direction of theoptical axis from the first group's lens to the prism).

[0013] In the above embodiment, it is possible to adopt an embodimentthat satisfies the following conditional formulas (1) and (2):

0.25<|fw/f1|<0.7   (1)

ν1>40   (2)

[0014] where f1 is the focal length of the first lens group, fw is thefocal length of the total lens system at the wide-angle end, and ν1 isthe Abbe number of the first lens group's lens. According to thisembodiment, if the value of |fw/f1| in the conditional formula (1)exceeds its lower limit, the refractive power of the lens of the firstlens group becomes too small, so that a necessary back focus cannot beachieved; on the other hand, if it exceeds the upper limit, the backfocus becomes too large, so that it becomes difficult to make the unitsmaller as well as to correct astigmatism and coma aberrations.Therefore, by satisfying the conditional formula (1), a better opticalcharacteristic and size reduction can be achieved. Also, by satisfyingthe conditional formula (2), lateral chromatic aberration can becorrected appropriately.

[0015] In the above embodiment, it is possible to adopt an embodimentthat satisfies the following conditional formulas (3):

0.1<f3/|f2|<0.8   (3)

[0016] where f2 is the focal length of the second lens group, and f3 isthe focal length of the third lens group. According to this embodiment,if the value of f3/|f2| in the conditional formula (3) exceeds the lowerlimit, it becomes difficult to achieve a zoom ratio of approximately 2;on the other hand, if it exceeds the upper limit, the back focus becomestoo large and the most outward entrance axis moves away from the opticalaxis at the wide-angle end, so that the first lens group's lens becomestoo large and makes it impossible to reduce the unit's size. Therefore,by satisfying the conditional formula (3), a zoom ratio of approximately2, a better optical characteristic and size reduction can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a drawing showing an embodiment of a zoom lens accordingto the present invention.

[0018] FIGS. 2(a) and (b) show the side views of the zoom lens shown inFIG. 1 at its wide-angle end and telephoto end.

[0019]FIG. 3 is a perspective view of the zoom lens shown in FIG. 1.

[0020] FIGS. 4(a), (b), (c), and (d) show aberration charts of sphericalaberration, astigmatization, distortion, and lateral chromaticaberration at the wide-angle end of the zoom lens according to theembodiment of FIG. 1.

[0021] FIGS. 5(a), (b), (c), and (d) show aberration charts of sphericalaberration, astigmatization, distortion, and lateral chromaticaberration at a middle position of the zoom lens according to theembodiment of FIG. 1.

[0022] FIGS. 6(a), (b), (c), and (d) show aberration charts of sphericalaberration, astigmatization, distortion, and lateral chromaticaberration at a telephoto end of the zoom lens according to theembodiment of FIG. 1.

[0023]FIG. 7 is a drawing showing another embodiment of a zoom lensaccording to the present invention.

[0024] FIGS. 8 (a) and (b) show the side views of the zoom lens shown inFIG. 7 at its wide-angle end and telephoto end.

[0025] FIGS. 9 (a), (b), (c), and (d) show aberration charts ofspherical aberration, astigmatization, distortion, and lateral chromaticaberration at the pantographic end of the zoom lens according to theembodiment of FIG. 7.

[0026] FIGS. 10 (a), (b), (c), and (d) show aberration charts ofspherical aberration, astigmatization, distortion, and lateral chromaticaberration at a middle position of the zoom lens according to theembodiment of FIG. 7.

[0027] FIGS. 11(a), (b), (c), and (d) show aberration charts ofspherical aberration, astigmatization, distortion, and lateral chromaticaberration at a telephoto end of the zoom lens according to theembodiment of FIG. 7.

[0028]FIG. 12 is a drawing showing another embodiment of a zoom lensaccording to the present invention.

[0029] FIGS. 13(a) and (b) show the side views of the zoom lens shown inFIG. 12 at its wide-angle end and telephoto end.

[0030] FIGS. 14(a), (b), (c), and (d) show aberration charts ofspherical aberration, astigmatization, distortion, and lateral chromaticaberration at the pantographic end of the zoom lens according to theembodiment of FIG. 12.

[0031] FIGS. 15(a), (b), (c), and (d) show aberration charts ofspherical aberration, astigmatization, distortion, and lateral chromaticaberration at a middle position of the zoom lens according to theembodiment of FIG. 12.

[0032] FIGS. 16(a), (b), (c), and (d) show aberration charts ofspherical aberration, astigmatization, distortion, and lateral chromaticaberration at a telephoto end of the zoom lens according to theembodiment of FIG. 12.

[0033]FIG. 17 is a drawing showing yet another embodiment of a zoom lensaccording to the present invention.

[0034] FIGS. 18(a) and (b) show the side views of the zoom lens shown inFIG. 17 at its wide-angle end and telephoto end.

[0035] FIGS. 19(a), (b), (c), and (d) show aberration charts ofspherical aberration, astigmatization, distortion, and lateral chromaticaberration at the wide-angle end of the zoom lens according to theembodiment of FIG. 17.

[0036] FIGS. 20(a), (b), (c), and (d) show aberration charts ofspherical aberration, astigmatization, distortion, and lateral chromaticaberration at a middle position of the zoom lens according to theembodiment of FIG. 17.

[0037] FIGS. 21(a), (b), (c), (d) show aberration charts of sphericalaberration, astigmatization, distortion, and lateral chromaticaberration at a telephoto end of the zoom lens according to theembodiment of FIG. 17.

DESCRIPTION OF NUMERICAL KEYS USED IN THE DRAWINGS

[0038] I First lens group

[0039] II Second lens group

[0040] III Third lens group

[0041]1, 11, 11″ Lens (first lens group)

[0042]2, 12, 12″ Prism (first lens group)

[0043]2 a, 12 a Entrance surface

[0044]2 b, 12 b Exit surface

[0045] L1 Entrance axis

[0046] L2 Exit axis

[0047]3. 13, 13″ Lens (second lens group)

[0048]4, 14, 14′, 14″ Lens (third lens group)

[0049]5, 15, 15″ Lens (third lens group)

[0050]6, 16, 16′, 16″ Lens (third lens group)

[0051]7, 18 Glass filter

[0052]8, 19 Aperture stop

[0053]17, 17′, 17″ Lens (third lens group)

[0054] D1-D16 Surface distance on optical axis

[0055] R1-R6, R8-R17 Curvature radius

[0056] S1-S17 Surface

[0057] Preferred Embodiment

[0058] A preferred embodiment of the present invention is describedbelow referring to the accompanying drawings.

[0059]FIG. 1 through FIG. 3 show an embodiment of a zoom lens accordingto the present invention, wherein FIG. 1 shows its basic embodiment,FIGS. 2(a) and (b) show a view of the positional relations at thewide-angle and at the telephoto end, and FIG. 3 is a perspective view ofthe embodiment.

[0060] In this zoom lens, a first lens group (I) that has a negativerefractive power as a whole, a second lens group (II) that has anegative refractive power as a whole and a third lens group (III) thathas a positive refractive power as a whole are laid out in that orderfrom the object side to the image side.

[0061] The first lens group (I) consists of a lens 1 that has a negativerefractive power and a prism 2 that changes the light path. The secondlens group (II) consists of a lens 3 that has a negative refractivepower. The third lens group (III) consists of a lens 4 that has apositive refractive power, a lens 5 that has a negative refractivepower, and a lens 6 that has a positive refractive power.

[0062] The lenses and the prisms that constitute the first lens group(1), the second lens group (II), and the third lens group (III) are allmade of resin materials. As they are made of resin materials, they arelight and inexpensive.

[0063] In the above embodiment, a glass filter 7 such as an infrared cutfilter or a low pass filter is provided on the image plane side relativeto lens 6 of the third lens group (III), and an aperture stop 8 isprovided between the second lens group (II) and the third lens group(III), i.e., between lens 3 and lens 4. Since aperture 8 is located inthe position as mentioned above, it is possible to make the lens groupsarrange on both sides of it to have approximately equal outer diameters,thus reducing the size as a whole.

[0064] In the above embodiment, the third lens group (III) moves fromthe image plane side to the object side, in other words, from thewide-angle end shown in FIG. 2(a) to the telephoto end as shown in FIG.2(b) to perform the zooming operation while the second lens group (II)moves to correct the image plane change caused by the zooming operation.Since the depth dimension D of the lens and the lateral total length Hof the lens (distance from prism 2 of the first lens group (I) to theimage surface) are unchanged during the zooming operation, it can beeasily mounted on cellular telephones, portable information terminalsand the like where the mounting spaces are limited.

[0065] The focal length of the first lens group (I) is denoted f1, thefocal length of the second lens group (II) is f2, the focal length ofthe third lens group (III) is f3, the focal length of the total lenssystem at the wide-angle end is fw, the focal length of the total lenssystem at the telephoto end is ft, and the focal length of the totallens system in the middle range is fm.

[0066] The surfaces of lens 1, prism 2, and lens 3 through lens 6 aredenoted Si (i=1-6, 8-13), the curvature radius of each surface Si is Ri(i=1-6, 8-13), the refractive ratio relative to line “d” is Ni, and theAbbe number is vi (i=1-6) as shown in FIG. 1.

[0067] As to glass filter 7, its surfaces are denoted Si (i=14, 15), thecurvature radius of surface Si is Ri (i=14, 15), the refractive ratiorelative to line “d” is N7, and the Abbe number is ν7. Further, eachspace (thickness, air gap) located between lens 1 and glass filter 7along the optical axis is denoted Di (i=1-14).

[0068] In prism 2, its entrance surface 2 a and exit surface 2 b areformed in rectangular shapes that are oblong in a directionperpendicular to a plane that contains entrance axis L1 and exit axisL2. In this case, the direction of the longer side of prism 2 and thedirection of the longer side of image pickup device (image surface)coincide with each other. As a result, the depth dimension D in theentrance axis L1 direction of the first lens group (I), i.e., the zoomlens, can be reduced, thus making the unit thinner.

[0069] A surface S2 with a smaller curvature radius between a surface S1of the object side of lens 1 and surface S2 of image plane side isformed as an aspherical surface, wherein this aspherical surface isformed in such a way that its negative refractive power weakens towardthe periphery. As a result, corrections of various aberrations, inparticular, correction of distortion, can be achieved.

[0070] A surface S8 on the object side of lens 4, a surface S11 on theimage plane side, of lens 5, and a surface S12 on the object side oflens 6 are formed as aspherical surfaces. Consequently, variousaberrations can be adjusted in a good balance, and spherical aberrationscan be corrected suitably, especially by forming surface S8 as anaspherical surface.

[0071] An aspherical surface can be expressed in the following formula:

Z=Cy ²/[1+(1−εC ² Y ²)^(1/2) ]+Dy ⁴ +Ey ⁶ +Fy ⁸ +Gy ¹⁰,

[0072] wherein Z is the distance from the vertex of the asphericalsurface to a point on the aspherical surface whose height from theoptical axis X is y; y is the height from the optical axis; C is theratio of curvature (1/R) at the vertex of the aspherical surface; ε isthe conical constant, and D, E, F, and G are aspherical coefficients.

[0073] In the above embodiment, the first lens group (I) is formed tosatisfy the following two formulas:

0.25<|fw/f1|<0.7, and   (1)

ν1>40,   (2)

[0074] where f1 is the focal length of the first lens group, fw is thefocal length of the total lens system at the wide-angle end, and ν1 isthe Abbe number of the lens of the first lens group (I).

[0075] The conditional formula (1) defines the ratio of an appropriatefocal length for the first lens group (I), where if the ratio exceedsthe upper limit, the back focus becomes too large, so that it becomesdifficult to make the unit smaller as well as to correct astigmatism andcoma aberrations; on the other hand if it exceeds its lower limit, therefractive power of lens 1 becomes too small, so that it becomesdifficult to secure a necessary back focus. In other words, it ispossible to achieve a satisfactory, optical capability and reduce thesize of the unit by satisfying this conditional formula (I).

[0076] The conditional formula (2) defines the Abbe number of lens 1that constitutes the first lens group (I), where if Abbe number is lessthan the lower limit it becomes difficult to correct the lateralchromatic aberration. In other words, by satisfying the conditionalformula (2), lateral chromatic aberration can be correctedappropriately.

[0077] Also, in the above embodiment, the second lens group (II) and thethird tens group (III) are constituted to satisfy the following formula:

0.1<f3/|f2|<0.8   (3)

[0078] (where f2 denotes the focal length of the second lens group, andf3 denotes the focal length of the third lens group.)

[0079] This conditional formula (3) defines an appropriate ratio betweenthe focal lengths of the second lens group (II) and the third lens group(III), where if it exceeds its lower limit, it becomes difficult toachieve a zoom ratio of approximately 2; on the other hand, if the ratioexceeds the upper limit, the back focus becomes too larger the outermostentrance axis moves away from the optical axis at the wide-angle end andmakes the lens of the first group too large, so that it becomesdifficult to make the unit smaller. Therefore, by satisfying theconditional formula (3), a zoom ratio of approximately 2, a betteroptical characteristic and size reduction can be achieved.

[0080] As an example using specific numerical values of the aboveembodiment, an embodiment 1 will be shown below. Table 1 shows the majordimensions of embodiment 1, Table 2 shows various numerical data (setupvalues), Table 3 shows numerical values of the aspheric surfaces, andTable 4 shows the focal length of the entire lens “f” (fw at thewide-angle end, fm at the middle position, and ft at the telephoto end)as well as numerical data concerning the spacing between the surfaces onthe axis D4, D6 and D13 at the wide-angle end, middle position, andtelephoto end specifically. In this example, the numerical data of theconditional formulas (1), (2) and (3) are: |fw/f1|=0.556 (fw=3.350 mm,f1=−6.023 mm), ν1=56.4, and f3/|f2|=0.158 (f2=−43.986 mm, f3=6.935 mm).

[0081]FIGS. 4a-4 d, FIGS. 5a-5 d and FIGS. 6a-6 d are the aberrationcharts of spherical aberration, astigmatic aberration, distortion, andlateral chromatic aberration at the wide-angle end, middle position, andtelephoto end respectively. In FIG. 4 through FIG. 6, FIG. 9 throughFIG. 11, FIG. 14 through FIG. 16 and FIGS. 19 through 21, “d” denotesthe aberration due to “d” line, “F” denotes the aberration due to “F”line, and “c” denotes the aberration due to “c” line, while SC denotesthe amount of dissatisfaction of the sine condition, DS denotes theaberration on the sagittal plane, and DT denotes the aberration of themeridional plane. TABLE 1 Total lateral length Object distance (prism toimage (mm) Infinity (∞) plane) mm 27.70 Focal length (mm) 3.35˜4.75˜Back focus (air 6.45˜8.75˜ 6.43 conversion) (mm) 11.03 F number2.89˜3.60˜ Angle of view (2ω) 61.3°˜43.1°˜ 4.39 31.9° Total lens length30.65 Focal length f1 −6.023 (front of lens 1 to (mm) image surface)(mm) Thickness of first 7.65 Wide-angle end 3.350 lens group (depth)focal length fw (mm) (mm) Thickness of 1.25 Focal length f2 −43.986second lens group (mm) (mm) Thickness of third 8.20 Focal length f36.935 lens group (mm) (mm)

[0082] TABLE 2 Curvature Refractive radius index Surface (mm) Distance(mm) (“d” line) Abbe number S1 R1 −32.751 D1 1.250 N1 1.50914 ν1 56.4S2* R2 3.427 D2 1.700 S3 R3 ∞ D3 4.700 N2 1.58385 ν2 30.3 S4 R4 ∞ D4variable S5 R5 −45.000 D5 1.250 N3 1.50914 ν3 56.4 S6 R6 45.000 D6variable S7 Aperture stop D7 0.000 S8* R8 4.800 D8 3.000 N4 1.50914 ν456.4 S9 R9 −8.084 D9 0.800 S10 R10 −39.076 D10 1.500 N5 1.58385 ν5 30.3S11* R11 20.910 D11 0.900 S12* R12 18.039 D12 2.000 N6 1.50914 ν6 56.4S13 R13 −73.116 D13 variable S14 R14 ∞ D14 1.200 N7 1.51680 ν7 64.2 S15R15 ∞

[0083] TABLE 3 Aspheric coefficient Numerical data S2 surface ε   0.5130000 D −0.6882592 × 10⁻³ E   0.6217665 × 10⁻⁵ F   0.1615279 ×10⁻⁵ G −0.3138584 × 10⁻⁶ S8 surface ε  −1.0000000 D   0.5790936 × 10⁻³ E  0.5066817 × 10⁻⁴ F −0.8724338 × 10⁻⁵ G −0.1568151 × 10⁻⁵ S11 ε−15.6000000 surface D   0.1230093 × 10⁻³ E −0.1160219 × 10⁻³ F−0.1716015 × 10⁻⁴ G −0.9113209 × 10⁻⁶ S12 ε −27.0000000 surface D−0.1450770 × 10⁻² E −0.2387584 × 10⁻³ F −0.1219637 × 10⁻⁴ G −0.4467548 ×10⁻⁶

[0084] TABLE 4 Middle Wide-angle end position Telephoto end f (mm) 3.354.75 6.43 (fw) (fm) (ft) D4 (mm) 1.000 2.982 1.318 D6 (mm) 5.700 1.4130.800 D13 (mm) 4.655 6.960 9.237

[0085] In the above embodiment 1, lens depth D (lens 1 to prism 2) is7.65 mm, total lateral lens length (prism 2 to image surface) H when itis in use is 27.70 mm, total lens length (front S1 of lens 1 to imagesurface) is 30.65 mm, back focus (air equivalent) is 6.45 mm-11.03 mm, Fnumber is 2.89-4.39, and angle of view (2ω) is 61.3°-31.9°, thusproviding a compact, thin, and a high optical capability lens with allaberrations suitably corrected.

[0086]FIG. 7 and FIG. 8 show basic embodiments and views of zoom lens ofanother embodiment according to this invention. In this zoom lens, afirst lens group (I) that has a negative refractive power as a whole, asecond lens group (II) that has a negative refractive power as a wholeand a third lens group (III) that has a positive refractive power as awhole are laid out in that order from the object side to the image planeside as shown in FIG. 7.

[0087] The first lens group (I) consists of a lens 11 that has anegative refractive power and a prism 12 that changes the light path.The second lens group (II) consists of a lens 13 that has a negativerefractive power. The third lens group (III) consists of a lens 15 and alens 14 having a positive refractive power, a lens 16 that has anegative refractive power connected to lens 15, and a lens 17 that has apositive refractive power.

[0088] The first lens group (1), the second lens group (II), and thethird lens group (III) are formed to satisfy the aforementionedconditional formulas (1), (2) and (3). The lenses and the prisms thatconstitute them are partially made of glass, but primarily of plastics,so that they are light and inexpensive to manufacture.

[0089] In the above embodiment, a glass filter 18 such as an infraredcut filter or a low pass filter is provided on the image plane siderelative to lens 17 of the third lens group (III), and an aperture stop18 is provided between the second lens group (II) and the third lensgroup (III), i.e., between lens 13 and lens 14. Since aperture stop 18is located in the position as mentioned above, it is possible to makethe lens groups arrange on both sides of it to have approximately equalouter diameters, thus reducing the size as a whole.

[0090] In the above embodiment, the third lens group (III) moves fromthe image side to the object side, in other words, from the wide-angleend, shown in FIG. 8(a) to the telephoto end as shown in FIG. 8(b) toperform the zooming operation, while the second lens group (II) moves tocorrect the image plane change caused by the zooming operation. Sincethe depth dimension D of the lens and the lateral total length H of thelens (distance from prism 12 of the first lens group (I) to the imagesurface) are unchanged during the zooming operation, it can be easilymounted on cellular telephones, portable information terminals and thelike where the mounting spaces are limited.

[0091] The surfaces of lens 11, prism 12, lens 13 through lens 17 aredenoted Si (i=7-6, 8-15), the curvature radius of each surface Si is Ri(i=1-6, 8-15), the refractive ratio relative to line “d” is Ni, and theAbbe number is vi (i=1-7) as shown in FIG. 7.

[0092] As to glass filter 18, the surfaces are denoted Si (i=16, 17),the curvature radius of surface Si is Ri (i=16, 17), the refractiveindex relative to line “d” is N8, and the Abbe number is ν8. Further,each space (thickness, air gap) located between lens 11 and glass filter18 along the optical axis is denoted Di (i=1-16).

[0093] Since prism 12, similar to prism 2 in the aforementionedembodiment, has both entrance plane 12 a and exit plane 12 b formedrectangular in such a way that they are oblong in a directionperpendicular to a plane including entry axis L1 and exit axis L2 (seeFIG. 3), the depth dimension D in the direction of entrance axis L1 canbe minimized, thus making it possible to make the unit thinner.

[0094] Further in the above embodiment, a surface S2 with a smallercurvature radius among a surface S1 of the object side of lens 11 andsurface S2 of image plane side is formed as an aspherical surface,wherein this aspherical surface is formed in such, a way that itsnegative refractive power weakens toward the periphery. As a result,corrections of various aberrations, in particular, correction ofdistortion, can be achieved.

[0095] A surface S8 of the objective side of lens 14 is formed as anaspherical surface. Consequently, various aberrations can be adjusted ina good balance, and spherical aberrations in particular can be correctedsuitably. The aspherical surface is formed to satisfy the aforementionedformulas.

[0096] As an example using specific numerical values of the aboveembodiment, an embodiment 2 will be shown below. Table 5 shows the majordimensions of embodiment 2, Table 6 shows various numerical data (setupvalues), Table 7 shows numerical values of the aspheric surfaces, andTable 8 shows the focal length of the lens as a whole “f” (wide-angleend fw, middle position fm, and telephoto end ft) as well as numericaldata concerning the spacing between the surfaces on the axis D4, D6 andD15 at the wide-angle end, middle position, and telephoto endspecifically. In this example, the numerical data of the conditionalformulas (1), (2) and (3) are: |fw/f1|=0.441 (fw=3.350 mm, f1=−8.157mm), ν1=56.4, and f3/|f2=0.378(f2=−18.763 mm, f3=7.099 mm).

[0097]FIGS. 9a-9 d, 10 a-10 d and 11 a-11 d are the aberration charts ofspherical aberration, astigmatic aberration, distortion, and lateralchromatic aberration at the wide-angle end, middle position, andtelephoto end respectively. TABLE 5 Total lateral length Object distance(prism to image (mm) Infinity (∞) plane) mm 28.11 Focal length (mm)3.35˜4.75˜ Back focus (air 6.80˜9.18˜ 6.44 conversion) (mm) 11.51 Fnumber 2.86˜3.50˜ Angle of view (2ω) 61.3°˜43.0°˜ 4.22 31.8° Total lenslength 31.06 Focal length f1 −8.157 (front of lens 11 (mm) to imagesurface) (mm) Thickness of first 7.75 Wide-angle end 3.350 lens group(depth) focal length fw (mm) (mm) Thickness of 1.25 Focal length f2−18.763 second lens group (mm) (mm) Thickness of third 7.95 Focal lengthf3 7.099 lens group (mm) (mm)

[0098] TABLE 6 Curvature Refractive radius index Surface (mm)Distance(mm) (“d” line) Abbe number  S1 R1 −315.429 D1 1.250 N1 1.50914ν1 56.4 *S2 R2 4.214 D2 1.700  S3 R3 ∞ D3 4.800 N2 1.50914 ν2 56.4  S4R4 ∞ D4 variable  S5 R5 −7.520 D5 1.250 N3 1.50914 ν3 56.4  S6 R6−37.321 D6 variable  S7 Aperture stop D7 0.000 *S8 R8 6.026 D8 2.100 N41.58385 ν4 30.3  S9 R9 −9.646 D9 0.200  S10 R10 6.810 D10 1.850 N51.51680 ν5 64.2  S11 R11 −6.810 D11 0.000  S12 R12 −6.810 D12 0.800 N61.80518 ν6 25.5  S13 R13 4.447 D13 1.000  S14 R14 9.569 D14 2.000 N71.50914 ν7 56.4  S15 R15 −5.857 D15 variable  S16 R16 ∞ D16 1.200 N81.51680 □7 64.2  S17 R17 ∞

[0099] TABLE 7 Aspherical surface coefficient Numerical data S2 surfaceε   1.1419393 D −0.1399480 × 10⁻² E −0.3359319 × 10⁻⁴ F   0.4537005 ×10⁻⁵ G −0.8650274 × 10⁻⁶ S8 surface ε −0.2784433 D −0.5958167 × 10⁻³ E  0.6184371 × 10⁻⁴ F −0.2760339 × 10⁻⁵ G −0.9278021 × 10⁻⁶

[0100] TABLE 8 Middle Wide-angle end position Telephoto end f (mm) 3.354.75 6.44 (fw) (fm) (ft) D4 (mm) 1.200 2.323 1.253 D6 (mm) 5.700 2.2000.930 D15 (mm) 5.007 7.384 9.724 (Back focus 1.00 mm)

[0101] In the above embodiment 2, lens depth D (lens 11 to prism 12) is7.75 mm, total lateral lens length (prism 12 to image surface) H when itis in use is 28.11 mm, total lens length (front S1 of lens 11 to imagesurface) is 31.06 mm, back focus (air equivalent) is 6.80 mm-11.51 mm, Fnumber is 2.86-4.22, and angle of view (2ω) is 61.3°-31.8°, thusproviding a compact, thin, and a high optical capability lens with allaberrations suitably corrected.

[0102]FIG. 12 and FIG. 13 show basic constitutions and views of zoomlens of other embodiments according to this invention. This zoom lenshas an identical structure as those embodiments shown in FIG. 7 and FIG.8 except that the specifications of lens 14′, lens 16′ and lens 17′ aremodified.

[0103] As an example using specific numerical values of the aboveembodiment, an embodiment 3 will be shown below. Table 9 shows the majordimensions of embodiment 3, Table 10 shows various numerical data (setupvalues), Table 11 shows numerical values of the aspheric surfaces, andTable 12 shows the focal length of the lens as a whole “f” (wide-angleend fw, middle position fm, and telephoto end ft) as well as numericaldata concerning the spacing between the surfaces on the axis D4, D6 andD15 at the wide-angle end, middle position, and telephoto endspecifically. In this example, the numerical data of the conditionalformulas (1), (2) and (3) are: |fw/f1=0.441 (fw=3.350 mm, f1=−8.157 mm),ν1=56.4, and f3/|f2|=0.370(f2=−18.763 mm, f3=6.943 mm).

[0104]FIGS. 14a-14 d, 15 a-15 d and 16 a-d are the aberration charts ofspherical aberration, astigmatic aberration, distortion, and lateralchromatic aberration at the wide-angle end, middle position; andtelephoto end respectively. TABLE 9 Total lateral length Object distance(prism to image (mm) Infinity (∞) plane) mm 27.73 Focal length (mm)3.35˜4.75˜ Back focus (air 6.42˜8.74˜ 6.44 conversion) (mm) 11.03 Fnumber 2.86˜3.39˜ Angle of view (2ω) 62.01°˜43.1°˜ 4.10 31.8° Total lenslength 30.68 Focal length f1 −8.157 (front of lens 11 (mm) to imagesurface) (mm) Thickness of first 7.75 Wide-angle end 3.350 lens group(depth) focal length fw (mm) (mm) Thickness of 1.25 Focal length f2−18.763 second lens group (mm) (mm) Thickness of third 7.95 Focal lengthf3 6.943 lens group (mm) (mm)

[0105] TABLE 10 Curvature Refractive radius index Surface (mm)Distance(mm) (“d” line) Abbe number  S1 R1 −315.429 D1 1.250 N1 1.50914ν1 56.4 *S2 R2 4.214 D2 1.700  S3 R3 ∞ D3 4.800 N2 1.50914 ν2 56.4  S4R4 ∞ D4 variable  S5 R5 −7.520 D5 1.250 N3 1.50914 ν3 56.4  S6 R6−37.321 D6 variable  S7 Aperture stop D7 0.000 *S8 R8 6.687 D8 2.100 N41.68893 ν4 31.2  S9 R9 −11.062 D9 0.200  S10 R10 6.810 D10 1.850 N51.51680 ν5 64.2  S11 R11 −6.810 D11 0.000  S12 R12 6.810 D12 0.800 N61.80518 ν6 25.5  S13 R13 4.416 D13 1.000  S14 R14 10.599 D14 2.000 N71.50914 ν7 56.4  S15 R15 −6.099 D15 variable  S16 R16 ∞ D16 1.200 N81.51680 ν8 64.2  S17 R17 ∞

[0106] TABLE 11 Aspherical surface coefficient Numerical data S2 surfaceε   1.2078700 D −0.1696780 × 10⁻² E   0.7620015 × 10⁻⁴ F −0.6060053 ×10⁻⁵ G −0.6619714 × 10⁻⁶ S8 surface ε   0.0000000 D −0.6213306 × 10⁻³ E  0.8818258 × 10⁻⁴ F −0.5543206 × 10⁻⁵ G −0.1293282 × 10⁻⁵

[0107] TABLE 12 Middle Wide-angle end position Telephoto end f (mm) 3.354.75 6.44 (fw) (fm) (ft) D4 (mm) 1.200 2.299 1.252 D6 (mm) 5.700 2.2811.035 D15 (mm) 4.628 6.948 9.241

[0108] In the above embodiment 3, lens depth D (lens 11 to prism 12) is7.75 mm, total lateral lens length (prism 12 to image surface) H when itis in use is 27.73 mm, total lens length (front S1 of lens 11 to imagesurface) is 30.68 mm, back focus (air equivalent) is 6.42 mm-11.03 mm, Fnumber is 2.86-4.10, and angle of view (2ω) is 62.0°-31.8°, thusproviding a compact, thin, and a high optical capability lens with allaberrations suitably corrected.

[0109]FIG. 17 and FIG. 18 show basic constitutions and views of a zoomlens of yet another embodiment according to this invention. This zoomlens has an identical structure as those embodiments shown in FIG. 7 andFIG. 8 except that the specifications of lens 11″, lens 12″, lens 13″through lens 17″ are modified, lens 15″ and lens 16″ are separated, andan image side surface 13 of lens 16″ and an object side surface 14 oflens 17″ are formed aspherical.

[0110] As an example using specific numerical values of the aboveembodiment, an embodiment 4 will be shown below. Table 13 shows themajor dimensions of embodiment 4, Table 14 shows various numerical data(setup values), Table 15 shows numerical values of the asphericsurfaces, and Table 16 shows the focal length of the lens as a whole “f”(wide-angle end fw, middle position fm, and telephoto end ft) as well asnumerical data concerning the spacing between the surfaces on the axisD4, D6 and D15 at the wide-angle end, middle position, and telephoto endspecifically. In this example, the numerical data of the conditionalformulas (1), (2) and (3) are: |fw/f1=0.556 (fw=3.350 mm, f1=−6.023 mm),ν1=56.4, and f3/|f2|=0.157 (f2=−43.986 mm, f3=6.921 mm).

[0111]FIGS. 19a-19 d, 20 a-20 d and 21 a-d are the aberration charts ofspherical aberration, astigmatic aberration, distortion, and lateralchromatic aberration at the wide-angle end, middle position, andtelephoto end respectively. TABLE 13 Total lateral length Objectdistance (prism to image (mm) Infinity (∞) plane) mm 28.15 Focal length(mm) 3.35˜4.75˜ Back focus (air 5.59˜7.89˜ 6.44 conversion) (mm) 10.18 Fnumber 2.88˜3.53˜ Angle of view (2ω) 61.4°˜43.0°˜ 4.39 31.7° Total lenslength 31.10 Focal length f1 −6.023 (front of lens 11″ (mm) to imagesurface) (mm) Thickness of first 7.65 Wide-angle end 3.350 lens group(depth) focal length fw (mm) (mm) Thickness of 1.25 Focal length f2−43.986 second lens group (mm) (mm) Thickness of third 9.50 Focal lengthf3 6.921 lens group (mm) (mm)

[0112] TABLE 14 Curvature Refractive radius index Surface (mm)Distance(mm) (“d” line) Abbe number  S1 R1 −30.895 D1 1.250 N1 1.50914ν1 56.4 *S2 R2 3.451 D2 1.700  S3 R3 ∞ D3 4.700 N2 1.58385 ν2 30.3  S4R4 ∞ D4 variable  S5 R5 −45.000 D5 1.250 N3 1.50914 ν3 56.4  S6 R645.000 D6 variable  S7 Aperture stop D7 0.000 *S8 R8 7.694 D8 2.000 N41.50914 ν4 56.4  S9 R9 −21.108 D9 0.300  S10 R10 7.738 D10 2.000 N51.48749 ν5 70.4  S11 R11 −14.932 D11 0.800  S12 R12 −37.395 D12 1.500 N61.58385 ν6 30.3 *S13 R13 10.472 D13 0.900 *S14 R14 17.002 D14 2.000 N71.50914 ν7 56.4  S15 R15 −59.703 D15 variable  S16 R16 ∞ D16 1.200 N81.51680 ν8 64.2  S17 R17 ∞

[0113] TABLE 15 Aspherical surface coefficient Numerical data S2 surfaceε    0.5530000 D −0.9247500 × 10⁻³ E   0.4103685 × 10⁻⁴ F   0.2631008 ×10⁻⁵ G −0.3268380 × 10⁻⁶ S8 surface ε  −3.5000000 D   0.4864181 × 10⁻³ E  0.6721384 × 10⁻⁴ F −0.6822639 × 10⁻⁵ G −0.1395979 × 10⁻⁵ S13 ε−10.3000000 surface D −0.7456721 × 10⁻⁴ E −0.1483760 × 10⁻³ F −0.1886347× 10⁻⁴ G −0.9735793 × 10⁻⁶ S14 ε −65.0000000 surface D −0.1716089 × 10⁻²E −0.2455649 × 10⁻³ F −0.1227574 × 10⁻⁴ G −0.9496339 × 10⁻⁵

[0114] TABLE 16 Middle Wide-angle end position Telephoto end f (mm) 3.354.75 6.44 (fw) (fm) (ft) D4 (mm) 1.000 2.977 1.301 D6 (mm) 5.700 1.4230.814 D15 (mm) 3.800 6.100 8.385

[0115] In the above embodiment 4, lens depth D (lens 11″ to prism 12″)is 7.65 mm, total lateral lens length (prism 12″ to image surface) Hwhen it is in use is 28.15 mm, total lens length (front S1 of lens 11″to image surface) is 31.10 mm, back focus (air equivalent) is 5.59mm-10.18 mm, F number is 2.88-4.39, and angle of view (2ω) is61.4°-31.7°, thus providing a compact, thin, and a high opticalcapability lens with all aberrations suitably corrected.

What is claimed is:
 1. A zoom lens comprising: a first lens group havinga negative refractive power as a whole, a second lens group having anegative refractive power as a whole, and a third lens group having apositive refractive power as a whole, arranged in said order from objectside to image side, for zooming from a wide-angle end to a telephoto endby means of moving said third lens group from image plane side toobjection side as well as for correcting image plane changes required inaccordance with said zooming by moving said second lens group; whereinsaid first lens group consists of a lens having a negative refractivepower and a prism for changing a light path arranged in said order fromthe object side.
 2. A zoom lens claimed in claim 1 wherein, said secondlens group consists of a lens having a negative refractive power; and anaperture stop is provided between said second lens group and said thirdlens group.
 3. A zoom lens claimed in claim 1 wherein, said first lensgroup's lens has an aspherical surface.
 4. A zoom lens claimed in claim3 wherein, said aspherical surface is formed on a surface with a smallercurvature radius.
 5. A zoom lens claimed in claim 4 wherein, saidaspherical surface is formed to have a weaker negative refractive powerweakening toward its periphery.
 6. A zoom lens claimed in claim 1wherein, said third lens group has at least one lens with a positiverefractive power and at least one lens with a negative refractive power.7. A zoom lens claimed in claim 6 wherein, said third lens group has alens at a position closest to the object having a positive refractivepower and an aspherical surface at least on one side.
 8. A zoom lensclaimed in claim 1 wherein, the prism of said first lens group is formedto have an entrance surface and an exit surface both oblong in adirection perpendicular to a plane that includes an entrance axis and anexit axis.
 9. A zoom lens claimed in claim 1 that satisfies thefollowing equations (1) and (2): 0.25<|fw/f1|<0.7,   (1) ν1>40,   (2)where f1: focal length of the first lens group, fw: focal length of thetotal lens system at the wide-angle end, and ν1: Abbe number of thefirst lens group's lens.
 10. A zoom lens claimed in claim 1 thatsatisfies the following equation (3): 0.1<f3/|f2|<0.8,   (3) where f2:focal length of the second lens group, and f3: focal length of the thirdlens group.
 11. A zoom lens claimed in claim 2 wherein, said first lensgroup's lens has an aspherical surface.
 12. A zoom lens claimed in claim2 wherein, said third lens group has at least one lens with a positiverefractive power and at least one lens with a negative refractive power.13. A zoom lens claimed in claim 3 wherein, said third lens group has atleast one lens with a positive refractive power and at least one lenswith a negative refractive power.
 14. A zoom lens claimed in claim 2wherein, the prism of said first lens group is formed to have anentrance surface and an exit surface both oblong in a directionperpendicular to a plane that includes an entrance axis and an exitaxis.
 15. A zoom lens claimed in claim 3 wherein, the prism of saidfirst lens group is formed to have an entrance surface and an exitsurface both oblong in a direction perpendicular to a plane thatincludes an entrance axis and an exit axis.
 16. A zoom lens claimed inclaim 2 that satisfies the following equations (1) and (2):0.25<|fw/f|<0.7,   (1) ν1>40,   (2) where f1: focal length of the firstlens group, fw: focal length of the total lens system at the wide-angleend, and ν1: Abbe number of the first lens group's lens.
 17. A zoom lensclaimed in claim 3 that satisfies the following equations (1) and (2):0.25<|fw/f1|<0.7,   (1) ν1>40,   (2) where f1: focal length of the firstlens group, fw: focal length of the total lens system at the wide-angleend, and ν1: Abbe number of the first lens group's lens.
 18. A zoom lensclaimed in claim 2 that satisfies the following equation (3):0.1<f3/|f2|<0.8,   (3) where f2: focal length of the second lens group,and f3: focal length of the third lens group.
 19. A zoom lens claimed inclaim 3 that satisfies the following equation (3): 0.1<f3/|f2|<0.8,  (3) where f2: focal length of the second lens group, and f3: focallength of the third lens group.
 20. A zoom lens claimed in claim 9 thatsatisfies the following equation (3): 0.1<f3/|f2|<0.8,   (3) where f2:focal length of the second lens group, and f3: focal length of the thirdlens group.